Black poop is usually caused by something you ate, drank, or took as a supplement or medication. But in some cases, it signals bleeding in the upper digestive tract, which needs medical attention. The key distinction is whether the color change has a clear, harmless explanation or whether it came on unexpectedly alongside other symptoms.
Common Harmless Causes
The most frequent reason for black stool is something you swallowed. Iron supplements are one of the top culprits, and they can darken your stool within 24 to 48 hours of taking a dose. The color change lasts about one to two days after each dose and is completely normal. If you recently started an iron supplement and noticed the change, that’s almost certainly the explanation.
Several foods and over-the-counter products do the same thing. Black licorice, blueberries, blood sausage, and activated charcoal can all turn stool noticeably dark. Bismuth-based stomach remedies like Pepto-Bismol are another well-known cause. When the bismuth in these products meets trace amounts of sulfur in your saliva and digestive system, they combine to form bismuth sulfide, a harmless black substance. This reaction can also temporarily darken your tongue.
When Black Stool Means Bleeding
Black stool that you can’t trace to food, supplements, or medication may be a sign of bleeding somewhere in your upper digestive tract, meaning the esophagus, stomach, or the first section of the small intestine. This type of stool has a distinct appearance: it’s not just dark but tarry, sticky, and has an unusually strong, foul smell. Doctors call this melena.
The color comes from a chemical change. When blood is exposed to stomach acid and digestive enzymes as it moves through the GI tract, the hemoglobin in red blood cells breaks down and turns from red to dark brown or black. By the time it reaches the toilet, it looks nothing like fresh blood. This is different from bright red blood in stool, which typically comes from bleeding lower in the digestive tract, closer to the rectum.
Common causes of upper GI bleeding include stomach ulcers, inflammation of the stomach lining, enlarged veins in the esophagus, and small tears near the junction of the esophagus and stomach. Heavy use of anti-inflammatory painkillers (like ibuprofen or aspirin) is a well-known risk factor for stomach ulcers that bleed.
Warning Signs That Need Immediate Attention
If your black stool is tarry and sticky, and you haven’t taken iron, bismuth products, or eaten anything that could explain it, that warrants a call to your doctor. But certain combinations of symptoms mean you should go to an emergency room rather than waiting for an appointment:
- Vomiting blood or vomit that looks like dark coffee grounds
- Dizziness, weakness, or lightheadedness
- Heart palpitations or shortness of breath
- Several days of black, tarry stool
These signs suggest significant blood loss. Feeling faint or having a racing heart means your body is struggling to compensate for the volume of blood you’ve lost, and that situation can deteriorate quickly.
How Doctors Find the Cause
If bleeding is suspected, your doctor will start with your medical history and a physical exam. A stool test can confirm whether there’s hidden blood in the stool that isn’t visible to the naked eye. Blood tests check for anemia (low red blood cell counts), how well your blood clots, and how your liver is functioning, since liver disease can contribute to GI bleeding.
The most direct way to find the source of bleeding is an upper endoscopy, where a thin, flexible tube with a camera is guided down the throat to visually inspect the esophagus, stomach, and upper small intestine. If the source isn’t found there, a colonoscopy examines the lower digestive tract. For harder-to-reach areas of the small intestine, you may be asked to swallow a vitamin-sized capsule containing a tiny camera that takes thousands of pictures as it passes through your system.
In many cases, if a bleeding site is found during an endoscopy, the doctor can treat it during the same procedure, stopping the bleed with heat, clips, or medication applied directly to the spot.
Black Stool in Newborns
If you’re a new parent, black stool in your baby’s first few days of life is perfectly normal. Newborns pass a thick, dark, tar-like substance called meconium, which is made up of materials swallowed during development in the womb. This typically appears in the diaper for a couple of days after birth. As the baby starts feeding, stool transitions to a yellowish-green color. Black stool that reappears after this transition period, however, is worth bringing up with your pediatrician.
How to Tell the Difference at Home
The simplest first step is to think back over the last 48 hours. Did you take iron, Pepto-Bismol, or activated charcoal? Did you eat a large amount of blueberries, black licorice, or dark-colored foods? If yes, that’s very likely the explanation, and the color should return to normal once you stop consuming whatever caused it.
If none of those apply, pay attention to the texture and smell. Stool that’s simply dark brown or black but otherwise looks and smells normal is less concerning than stool that’s sticky, tarry, and unusually foul-smelling. That tarry quality is the hallmark of digested blood and is the signal that something more serious may be happening. Even a single episode of true tarry stool, especially if you’re also feeling unusually tired or lightheaded, is worth getting checked out promptly.